1. Field of the Invention
The present invention relates to a dielectric filter comprising a plurality of dielectric resonators juxtaposed in a dielectric ceramic block.
2. Prior Art
There are known dielectric filters of the type comprising a rectangularly parallelepipedic dielectric ceramic block, three or more than three resonators each of which includes a through hole extending longitudinally in the dielectric ceramic block and an inner conductor provided on a peripheral wall of the through hole, and an outer conductor covering the substantial portion of the outer peripheral surface of the dielectric ceramic block, except one end surface of the dielectric ceramic block which forms an open-circuiting end surface on which one opening ends of the through holes are arranged, the other opening ends thereof being arranged on a short-circuiting surface opposite to the open-circuiting end surface of the dielectric ceramic block.
FIGS. 1 and 2 of the accompanying drawings show a conventional dielectric filter of the above identified type with three resonators, a central resonator A and two outer resonators B in a dielectric ceramic block, wherein an open-circuiting end surface of the the dielectric ceramic block is provided with a conductor pattern C which is extended from the edge of the opening on the open-circuiting end surface of the dielectric ceramic block toward the edges of the openings of the adjacently located outer resonators B. One end of the conductor pattern C is connected to the inner conductor of the central resonator A and the other end or free end of the the conductor pattern C is separated from the edges of the openings of the outer resonators B by respective insulating gaps G in order to realize a capacitive interstage coupling for coupling the resonators with each other.
While the resonators A and B are normally made to have a length equal to .lambda./4 or a quarter of a specified resonant frequency, the conductor pattern C formed as an extension of the central resonator A increases the effective resonant length of the resonator A to lower its resonant frequency and make it disagree with those of the outer resonators B. Consequently, such a dielectric filter does not provide a satisfactory filtering effect.
This problem may be overcome by the provision of a recess D in a central area of the bottom of the dielectric ceramic block as shown in FIGS. 1 and 2, which is the short-circuiting end of the dielectric ceramic block, to make the resonant length of the central resonator A shorter than that of the outer resonators B and shift the resonant frequency of the resonator A upward in advance in order to compensate the lowered resonant frequency of the central resonator A and make the resonant frequencies of all the resonators consequently agree with each other.
It should be noted that such a conventional dielectric filter is provided with a conductive film E on the short-circuiting end surface of the dielectric ceramic block, and the conductive film E is connected to the edges of the openings of the resonators on that side. The conductive film E is typically prepared by screen printing which is adapted to mass production. However, with the configuration of the dielectric filter of FIGS. 1 and 2 having a recess D formed in a central area of the short-circuiting end surface, the screen printing technique cannot feasibly be used and the conductive film E has to be formed by applying a conductive material to that side by means of a brush at the cost of manufacturing efficiency. In short, such a configuration is not adapted to mass production.
It is, therefore, the object of the present invention to provide a dielectric filter that can provide a necessary coupling capacitance without requiring the formation of a recess on a short-circuiting end surface of a dielectric ceramic block to make the resonant frequencies of the resonators agree with each other.